Microstructure parameters affecting interfacial adhesion of thermal barrier coatings by the EB-PVD method

Current numerical approaches in modeling the intrinsic failure of TBC relies largely on the notion that spallation occurs when the accumulating strain energy stored in the coating exceeds a fixed critical value resembling the interfacial adhesion. If this is to be entirely correct, one would expect this critical value of interfacial adhesion varies with different materials, but stays independent of their thermal exposure history. In this study, a unique cross-sectional indentation technique was developed to quantitatively characterize the adhesion of oxide-bond coat interface among 5 systematically prepared material systems. The results not only re-confirmed that interfacial adhesion is a material specific property in general, but more importantly, strongly implied that the adhesion is dynamic, in particular with time and temperature. With an aim of further understanding the dynamics (i.e. establishing correlations between time and temperature dependent microstructure effects), parameters such as the oxide growth rate, rumpling of the oxide-bond coat interface, and phase transformation of bond coat were studied as a function of thermal exposure. It has been clearly indicated here that the oxide-bond coat interfacial adhesion bears strong dependency on the phase distribution of the bond coats and TGO growth rate, while receiving little influence from TGO rumpling and residual stress.